Field of the Invention
This disclosure relates to a sheet feeding apparatus and image forming apparatus capable of feeding a sheet having an uneven thickness such as an envelope.
Description of the Related Art
An image forming apparatus such as a copier, facsimile apparatus, printer, or multi-function machine has a sheet feeding apparatus that separately feeds sheets on a one-by-one basis stacked on a sheet stacking portion toward an image forming portion. Recently, as an on-demand digital printing market extends, there is a growing printing need for a large variety of sheets in addition to conventional standard-size cut sheets.
For example, an image forming apparatus performs printing on an envelope, an emboss sheet with an uneven surface decorated with characters or pictures, or a preprinted sheet on which a toner image or ink has been printed in advance partly. In the case of envelopes, when a large number of envelopes are stacked on a sheet stacking portion of an image forming apparatus to print addresses or the like, the thickness of each envelope varies partly between an overlapping part of a bag portion and a flap portion such as a turnup of each envelope, thereby causing a large difference in the thickness of an envelope bundle partly. In addition, in the case of partly embossed sheets or preprinted sheets on which an image toner or ink has been printed partly as a thick layer, a large difference may be caused in the thickness of a sheet bundle partly, as in the case of envelopes to be set on the sheet stacking portion.
When a large number of such sheets having partly different thickness are stacked on the sheet stacking portion and fed on a one-by-one basis, height differences are caused on the upper surface of a stacked sheet bundle. The height differences of the upper surface of the sheet bundle make a contact pressure (feeding pressure) between a feeding roller for feeding the sheets and the topmost sheet uneven. When the feeding roller feeds the sheets in a state in which the contact pressure is uneven, a sheet feeding failure such as oblique conveyance or duplicate feeding may be caused. Accordingly, for example, Japanese Patent Laid-open No. 2006-103949 proposes a sheet feeding cassette that can reduce the height differences of the upper surface of the stacked sheet bundle by providing a bottom of the sheet bundle partly on a feeding tray on which the sheet bundle is stacked.
However, in the above configuration, although a sheet with an uneven thickness such as an envelope can be fed from the sheet feeding cassette smoothly, the maximum number of sheets that can be fed from the sheet feeding cassette for each replenishment is approximately 250 to 500.
To meet a recent need for continuously feeding and printing a large number of sheets at high speed, there has been a large capacity sheet feeding apparatus that has a lifter mechanism for elevating or lowering a sheet stacking portion on which sheets are stacked while keeping its attitude horizontal to feed the topmost sheet sequentially. This large capacity sheet feeding apparatus can replenish one thousand to several thousand sheets to the sheet stacking portion at a time and feed a large number of sheets continuously. In this sheet feeding apparatus, the lifter mechanism lifts the sheet stacking portion based on detection of the upper surface of the sheets by a sheet height detection sensor to keep the upper surface of stacked sheets at a height at which a sheet feed unit such as a feeding roller can feed the sheets.
However, when sheets with an uneven thickness such as envelopes or emboss sheets are stacked on the sheet stacking portion, the sheets may be stacked and fed in a state in which the flap portions of envelopes or thick portions of the sheets face the downstream side in a sheet feeding direction. In this case, on the upper surface of the stacked sheet bundle, the downstream side in the sheet feeding direction is higher and an upstream side is lower. Particularly in the large capacity sheet feeding apparatus, a large number of sheets are stacked, so the height difference between the downstream side and the upstream side in the sheet feeding direction on the upper surface of the stacked sheets becomes large. On the other hand, in the sheet feeding apparatus, there are cases where a sheet height detection sensor for detecting the upper surface of the sheets is disposed downstream in the sheet feeding direction and a sheet presence detection sensor for detecting presence or absence of sheets is disposed upstream of the sheet height detection sensor.
In the sheet feeding apparatus in which these detection sensors are disposed as described above, when the stacked sheets having a large height difference on the upper surface are continuously fed, “NO SHEET” may be detected erroneously even though sheets remain on the sheet stacking portion. Such erroneous detection occurs when the upstream side in sheet feeding direction is significantly lower than the downstream side on the upper surface of the stacked sheet bundle.
In a state in which the upper surface of the sheets is kept at a height at which the sheets can be fed based on detection by the sheet height detection sensor located downstream, if the upstream side is significantly lower than the downstream side on the upper surface of the sheets, the sheet presence detection sensor disposed upstream may not detect the upper surface of the sheets. Accordingly, a control portion erroneously detects “NO SHEET” based on a signal from the sheet presence detection sensor. Therefore, it may cause a disadvantage that a sheet feeding operation is stopped even though sheets remain on the sheet stacking portion.